A head up display emits light that reflects from the front windshield to be seen by the driver. The light appears to come from a virtual image in front of the driver and in front of the windshield. This type of head up display is currently commercially available.
Conventional head up displays create the virtual image by first using a display to create an image. Next, the light from the image is reflected from one or more mirrors. Next, the light from the mirrors is reflected from the windshield. The mirrors are designed and positioned relative to the display so that the light seen by the driver, which is reflected from the windshield, appears to come from a virtual image that is outside of the vehicle. The mirrors and display are typically contained in a package that occupies a volume beneath the top surface of the dashboard.
Current practice for automotive head up displays is to display symbols that are not positioned relative to objects seen by the driver. Examples include: vehicle speed, the speed limit, a warning that the vehicle is crossing a lane marker, and a warning that another vehicle is in the vehicle's blind spot.
FIG. 1 illustrates one embodiment of a head up display of the prior art, including a picture generation unit 28 and projection optics in the form of a mirror 30 that tilts. Mirror 30 reflects a light field from picture generation unit 28 onto a windshield 14, which further reflects the light field toward the eyes of a driver 24, who sees the light field as a virtual image disposed outside of windshield 14. In order to accommodate a range of driver heights, mirror 30 inside the head up display is tilted to adjust the apparent height of the virtual image seen by driver 24.
Typically, the tilt adjustment is manually set by the driver to enable the driver to see the virtual image. However, the mirror tilt adjustment changes the vertical position of the virtual image, as seen by the driver. This makes it difficult to accurately position symbols on the virtual image as seen by the driver.
Some aircraft use a head up display to display to the pilot symbols at an intended position relative to real world objects. Aircraft use a separate combiner from the windscreen of the aircraft. Consequently, they use a different optical projection method to create a virtual image.